Abstract: To indicate the microalloy effect of the rare earth element-Er on the alloys which could be strengthened by thermal treatment (precipitation consolidation, by ingot metallurgy six different compositions of Al-Zn-Mg alloys containing rare earth element Er were prepared and studied. The effects of Er on the microstructures and properties of Al-Zn-Mg alloys were studied through hardness test, tensile properties measurement, optical microscopy, X-ray diffraction analysis(XRD), scanning electron microscopy (SEM) and energy spectrum analysis. Experimental results show that the cast microstructures of Al-Zn-Mg alloys can be remarkably refined, and the dentrite structures eliminated, too. When the content of Er is up to w (Er)=0.7%, the dentrite structures of the alloy almost disappear, and the grains become very fine and equably distributed. As for the cool-rolled microstructure and the aged microstructure, the element Er has the same refining effects on them. The tensile strengh σ_b, and the yield strengh σ_0.2 have been increased significantly after adding Er to them, but the elongations decrease a little. When adding Er to the Al-Zn-Mg alloy, element Er reacts with element Al to form the Al₃Er phase. The refining of the microstructures and the strengthening of the alloy are relative to the formation and precipitation of the Al₃Er phase.